As flights were halted at Hong Kong’s airport, one of the world’s busiest, Chinese police were ominously assembling in nearby Shenzhen.

What to watch: “If the unthinkable happens and there is a violent crackdown in Hong Kong, then China’s relationships with the U.S. and its allies will likely get much worse,” writes Bill Bishop in his Sinocism newsletter. “I hate to be so negative but it does feel like we are approaching the precipice of something very worrisome.”

While the protests in Moscow haven’t matched the scale of Hong Kong's, the more than 50,000 who took to the streets on Saturday comprised the largest demonstration against the government since Vladimir Putin’s return to the presidency in 2012.

Some 2,400 people, including top opposition leaders, were arrested at previous demonstrations, which began after the barring of opposition candidates from municipal elections.

Alina Polyakova of the Brookings Institution says tensions were already running high in Moscow, so it took just "one little spark to incite something.”

She says falling standards of living across Russia over the last several years — reflected in smaller-scale protests over pensions and the environment — translated into greater shows of public support for the opposition than the Kremlin likely anticipated.

What to watch: Polyakova expects “an escalation of repression."

She says Putin can't afford to be perceived as weak, particularly since he already “looks like a lame duck to many people" in the face of constitutional term limits.

The big picture: Both China and Russia have accused the West of fomenting the unrest.

At least 7 people, many or all of them scientists, were killed last week when a missile test resulted in an explosion near Russia’s Arctic coast, per the NYT.

Little has been confirmed publicly about the explosion, except that it released radiation.

Between the lines: “The reference to radiation was striking — tests of missile engines don’t involve radiation. Well, with one exception: Last year, Russia announced it had tested a cruise missile powered by a nuclear reactor. ... NATO calls it the SSC-X-9 Skyfall,” Jeffrey Lewis writes for Foreign Policy.

That helps explain President Trump’s tweet today referring to a “ ‘Skyfall’ explosion,” which he said “has people worried about the air around the facility, and far beyond.”

Indeed, David Sanger and Andrew Kramer report in the Times that “the Russian government’s slow and secretive response has set off anxiety in nearby cities and towns — and attracted the attention of analysts in Washington and Europe who believe the explosion may offer a glimpse of technological weaknesses in Russia’s new arms program.”

Lewis is one such analyst. He writes that “a nuclear-powered cruise missile is an outrageous idea, one the United States long ago considered and rejected as a technical, strategic, and environmental nightmare."

"Vladimir Putin’s Russia, though, thinks differently.”

“The United States and Russia seem to be drifting into a new arms race, either out of some bizarre nostalgia or because no one can think of anything better to do.”

“When we think about the dangers of the arms race, we think about the possibility of a civilization-ending cataclysm. But even though the Cold War didn’t end in wide-scale catastrophe, it still resulted in a series of small-scale catastrophes for many of the people who lived it.”

Flashback: Macri was greeted enthusiastically by business and foreign investors when elected in 2015, but has overseen an economic recession and steep inflation, which he blames on his leftist predecessor, Cristina Fernández de Kirchner.

“In one poll, the most popular word Argentines used to describe Mr. Macri was useless. The top word for Mrs. Kirchner was corrupt. For Mr. Fernandez, who once criticized his running mate, it was flip-flopper,” the WSJ reports.

My weekend reading has me thinking about the relationship between American tourists and the Caribbean.

1. A death at a luxury resort in Anguilla “has riled the small island’s population and has raised uncomfortable questions about class, privilege and the deference shown to tourists,” the NYT’s Michael Wilson reports.